The object of this invention is a method for the production of conductive nickel oxide surfaces by chemical doping of the nickel oxide with alkali oxides, in particular for the use of the nickel in electrochemical applications.
In electrochemical processes, chemical reactions are regulated by an external electric current. Inside the electrochemical cells, the electrons must be transported by a conductive, stable and economical conductor. Nickel has thereby turned out to be an ideal material for the electrodes. But one disadvantage is the formation of nickel surfaces that are poor conductors or non-conductive when the electrodes are operated above the nickel hydroxide potential. In many processes, hydroxide formation occurs on account of the low level of this potential.
These poorly conducting or non-conducting hydroxide layers present a problem, for example, when pure nickel is used as the oxygen generation electrode for the electrolysis. But also in systems in which nickel is used in the form of a conductive fabric, an expanded metal mesh or a sheet that comes into contact with catalytically active material such as carbon, platinum-coated carbon etc., the isolating coating has a negative effect. For example, the hydroxide coatings also prevent an optimal current flow on oxygen consumption electrodes.
In zinc/air and nickel/metal-hydride batteries, oxygen cathodes for chlorine-alkali electrolysis and/or oxygen electrodes in alkaline fuel cells, for example, the efficiency of the entire system deteriorates as a result of ohmic losses on the surface of the nickel.
It is known that nickel surfaces can be roughened by mechanical methods to produce better electrical contact between nickel and other components of the electrode, such as activated carbon, for example. However, the initially rather low electrical resistance increases in operation, because the nickel surface becomes coated with non-conductive nickel hydroxide.
An additional method is the reduction of a complete electrode, which takes several hours. With nickel above all, which is in direct contact with carbon, the reduction not only results in the removal of the non-conducting surface, but also in a relatively stable bond between the metallic nickel and the carbon. One disadvantage of this method is that it is not possible, for example, to reduce the air electrode—which is made of activated carbon, manganese dioxide and nickel fabric—of a finished zinc/air battery in only a few hours at the hydrogen potential.
This method can be used in open systems, although the bonds thus formed between the nickel and carbon are not particularly stable. Especially in oxygen generation, the reduction must be repeated after no more than a month, because a new nickel hydroxide layer will have accumulated between the activated carbon and the nickel fabric.
It is known that low-conductivity nickel oxides exhibit a significant increase in conductivity with the addition of a low proportion of lithium oxide [P. J. Fensham, J. Amer. Soc. 76, 969 (1954) Löslichkeit von LiO2 (Solubility of LiO2)]. However, high temperatures are required for the application. For electro chemical applications, however, complicated nickel parts such as fabrics, expanded metal mesh or battery tanks are necessary, which cannot be exposed to high temperature loads, because otherwise they might be deformed.
The prior art also describes solutions for coating glass with conductive nickel oxide, in which lithium is added in measured quantities, thereby forming en electrically conductive coating. However, this method is also used in copiers and in industrial glass. One disadvantage of this process is again the high temperature, as described in DE 692 12 528.
In battery technology, the conductivity of the nickel is important both for the alkaline storage batteries of the nickel/cadmium type as well as for the nickel/metal-hydride type, as described in DE 697 21 136. In lithium batteries, the mounting of the lithium in nickel is likewise known. On this subject, see also DE 691 24 158.
In addition to this high-temperature process, the prior art also describes a low-temperature process in which an active nickel electrode is improved by a treatment in a mixture of KOH, NaOH, BaOH and hydrogen peroxide. In this case, however, the prior art also describes the treatment of an active electrode—and therefore not the treatment of pure metallic surfaces.